Killary Harbour & Mannin Bay

Overview

Killary Harbour

Killary Harbour is a fjord located in the west of Ireland in the heart of Connemara which forms a natural border between counties Galway and Mayo. It is 16 kilometres long and in the centre over 45 metres deep.

Entrance to Clifden Bay with the beacon on Errislanan marking the southern entrance of the bay. In the background are the Twelve Pins mountains of Connemara. The ruins of Clifden castle can also be seen in the centre of the photograph.

Mannin Bay lies to the southwest of Clifden and is best known for it's maerl sands.

Shaded Relief

The 3D appearance is achieved using software called Fledermaus. By using some vertical exaggeration, artificial sun-shading (usually as if there is a light source in the nw 315°) and colouring the depths using various colour maps, it is possible to highlight the subtle relief of the seabed. This helps us to quickly understand the variation in depths.

Backscatter

Multibeam Systems also collect additional information, including the strength of the acoustic signal (or return) from the seafloor. This is known as Backscatter. Differing seafloor types, such as mud, sand, gravel and rock will have different Backscatter values depending on the amount of energy they return to the sonar head. Rocky areas will typically have high returns while soft sediments like mud are more likely to absorb energy and have low Backscatter returns. These differing values are used to generate a grey-order image (i.e. dark for high returns, bright for low returns) of the seabed which can be used to examine the nature of the seafloor.

Ground Truthing/Seabed Sampling

Some seabed samples were taken in Killary Harbour by the Celtic Explorer in 2005 and by the Celtic Voyager in 2015 in Mannin Bay (Blue circles).

15 samples were taken by Aquafact in Mannin Bay in 2010 and 26 samples in Killary Harbour 2011. 16 samples were also taken by MERC Consultants around the Slyne Head peninsula in 2011.(Click image for more detail)

Geology

Mannin Bay Geology
The geology of the Mannin bay coastline is ancient and complex. Most of the coastline is comprised of a deformed group of metamorphic rocks known as the Dalradian, which have seen episodes of tectonic unrest in the past, resulting in much faulting and folding.

In the very north and southwest of the bay there are occurences of younger granite, an igneous rock that is also found in Connemara. The underwater expanse of craggy and fractured rock outcrop offshore, visible in the seabed data, seems to compiment the complexities of the geology onland.

Killary Harbour Geology
Known as “Ireland’s only Fjord”, Killary Harbour is a dramatic feature. The seabed data shows that it is deepest towards its entrance, becoming shallower inland towards the village of Leenane. It was carved by glaciers during the last Ice Age. The ancient bedrock geology to the north is comprised of sedimentary rocks known as greywackes, sandstones and shales. Mweelrea, Connaught’s highest mountain, is located on Killary Harbour’s north coast.

Shaded relief data of the Killary Harbour area shows regions of outcropping seabed separated by smooth seabed. The bedrock is extensively faulted and fractured. The image shows two digitised large scale faults orientated northwest – southeast. Numerous other smaller scale faults and fractures are present in the bedrock indicating a history of deformation.